This invention concerns the formation in spinnerets of holes or orifices of non-round cross-section having one axis or no axis of symmetry in the plane of the spinneret face; the holes or orifices being so formed as to eliminate non-axisymmetric emergence behavior in the spinning of inelastic materials, i.e., to eliminate "kneeing" of filaments as they are extruded from the spinning orifices. These "knees" have sometimes been called "bent filaments" or "dog legs".
In the textile art, the extrusion of filaments from orifices having essentially a round cross-section is known. Orifices of other cross-sectional configurations, however, have also been employed because the resulting extruded filaments have exhibited certain desirable and advantageous physical and aesthetic properties over the filaments extruded through round cross-sectioned orifices. The properties affected may concern resiliency and stiffness, bulk and cover, hand and the like; optical properties such as dullness, sparkle, brightness and the like; and yarn frictional properties and the like.
The extrusion of filaments from non-round cross-sections especially wherein the orifice configuration has only one axis or no axis of symmetry in the plane of the orifice cross-section, usually tends to be more difficult because of the propensity of filaments, when extruded through such orifices, to knee or form dog legs and drips or blobs.
A filament "knees" when the line of flow of the extruded filament from the orifice is bent out of the vertical back toward the spinneret face at an angle relative to the perpendicular to the spinneret face. In some instances the filament is bent to such extent that the filament forming material bends back and touches the spinneret face. This leaves a drip or blob of material on the spinneret face which can sometimes block a spinning orifice and interfere with filament formation. Sometimes such "kneeing" results in the coalescence of two or more adjacent filaments.
Some approaches toward elimination of kneeing of filaments that have been spun from spinneret orifices of noncircular cross-section taken by the prior art are shown in U.S. Pat. Nos. 3,640,670, 3,652,753 and 3,738,789.
U.S. Pat. No. 3,640,670 asserts for one of its embodiments that kneeing can be substantially eliminated in the use of T-shaped orifices by reversing the direction of the stem of the T so that the stem of the T points away from the center of a spinneret rather than toward the center of the spinneret. Another of the embodiments provides in a spinneret "split T" orifices wherein the crossbars and stem of the T are constructed by forming two rectangular orifices separated by a gap of such dimension that the resulting extrusions coalesce to form a single filament as though being spun from an integral T-shaped orifice. The resulting coalescence is due to the "Barus effect" because the extruded materials will expand at the exit and come into contact with each other. The more elastic the material being extruded the easier it is to utilize the Barus effect to cause coalescence because of the greater expansion of the material at the exit of the orifice. In U.S. Pat. No. 3,640,670, if one happens to select a T-shape which naturally knees toward the leg of the T and places it in a configuration such that the pattern is radial with all T legs pointing away from the spinneret center, then kneeing will be reduced. One is simply taking advantage of the decrease in melt viscosity, i.e., decreased resistance to flow, with increasing radius, i.e., a consequence of thermal instability. However, if one happens to choose a T shape which naturally knees toward the bar of the T and places it in the same configuration, kneeing will be more severe. Thus, opposite conclusions about the effect of the T orientation on the spinnerette face can be made, depending upon the T selected.
U.S. Pat. Nos. 3,652,753 and 3,738,789 together present still another approach, the first disclosing a process and the latter, a division of the first, disclosing a spinneret. These patents assert that kneeing can be controlled and virtually eliminated by constructing a T-shaped orifice so that the "extrusion factor" thereof as determined by the viscous resistance ratio of stem to crossbar is within a defined numerical range. The "viscous resistance" is defined as the ratio of pressure drop across the particular section of the orifice to the volume rate of flow through the orifice, and may be expressed as a function of the side wall dimensions of each rectangular segment of the T-shaped orifice. The two patents illustrate a T-shaped cross-section with "a" being the length of the crossbar section, "b" being the width of the crossbar section, "c" being the length of the leg or stem or tail portion and "d" being the width of the leg. The "viscous resistance" of the crossbar section of the orifice is thus expressed as a function of ab.sup.3, and the "viscous resistance" of the leg or stem or tail portion of the T-shaped orifice is expressed as a function of cd.sup.3. In the preferred embodiments, kneeing is said to be reduced through a T-shaped orifice wherein the crossbar and stem segments are essentially rectangular and wherein the stem segment is normal to the midpoint of the crossbar by constructing the orifice so that its ratio of ab.sup.3 /cd.sup.3 is about 0.65 to 0.90 or more, preferably about 0.75 to 0.80 and most preferably about 0.78.
In the ratio of ab.sup.3 /cd.sup.3 of the two U.S. Pat. Nos. 3,738,789 and 3,652,753, the numerator represents the resistance of flow offered by the rectangle making up the bar of the T, whereas the denominator represents the resistance to flow offered by the rectangle making up the leg of the T. This ratio is an indicator of the relative volumetric throughputs for the leg and bar of the T. It does not, however, take into account the interaction at the intersection of the rectangles. This means, therefore, that the ratio cannot be the only ratio. It can be shown that in the ratio as the dimension "b" (the width of the cross-bar section) approaches zero, the resulting orifice becomes a rectangle through which the extruded filament does not knee. Similarly, as the dimension "a" (the length of the crossbar section) approaches zero, the resulting orifice becomes a rectangle through which the extruded filament does not knee; and likewise, as the dimension "c" (the length of the leg) approaches zero, the resulting orifice becomes a rectangle through which the extruded filament does not knee. Thus, it would appear that several nonkneeing situations exist outside the specified range indicated in the two patents. On the other hand, it can also be shown that, for instance, when the normalized T dimensions are a = 1, b = 1 13/20, c = 6 and d = 9 13/20, the resulting extrusion factor in the ratio ab.sup.3 /cd.sup.3 is 0.84, which is within the claimed range of the patents but provides an orifice through which it has been found that the extruded filament severely knees.